1. Field of the Invention
This invention relates to a process for the precision cutting of a glass sheet and to a glass disk used for making a recording medium.
2. Description of the Related Art
There is known a method in which a glass sheet is marked with scratch lines by a glass cutter having an ultrahard alloy roll and is broken or severed along those lines to produce a glass sheet having a desired size. This method is called xe2x80x9cmechanical scribingxe2x80x9d. It is, however, known that mechanical scribing leaves oval shell-shaped chips discontinuously along the scratch lines and forms dust of glass scattering and adhering to the product surfaces. The shell-shaped chips are undesirable as they make a product of very low quality. The dust adheres to the product so strongly as not to be removable by pickling or any other way of cleaning, and requires a high cost of after-treatment. The method is basically carried out manually and is, therefore, low in productivity. Thus, it is a method which is not suitable for precision cutting, or mass production.
A method of cutting glass on a mass-production basis is proposed in WO93/20015 entitled xe2x80x9cSPLITTING OF NON-METALLIC MATERIALSxe2x80x9d. In the proposed method, a glass surface is marked with scratches, and heated by a laser beam so that a large amount of thermal stress may be produced therein. The glass is broken or severed if its thermal stress exceeds its breaking stress. The method is high in productivity, since it requires only small scratches to be made on glass and can be carried out automatically. The smaller the scratches, however, the sharper right-angled edges are formed between the glass surfaces and the broken or cut surfaces. They are dangerous for handling and require beveling. Moreover, the cut surfaces are not satisfactory in accuracy.
It is an object of this invention to provide a process which enables the highly accurate cutting of a glass sheet, while maintaining productivity.
According to a first aspect of this invention, there is provided a process for cutting a glass sheet, which comprises the steps of heat treating a glass sheet to be cut by applying at least one laser beam onto its surface and scanning the beam along a cutting line along which the sheet is to be cut, so that a sheet portion including the cutting line may be heated to a temperature equal to, or above its glass transition point, and then cooled to have a specific volume which is larger than that of the remaining portion; etching the heat-treated sheet to form a groove along the cutting line; and breaking the sheet along the groove.
A sheet portion including a cutting line is lowered in density by heat treatment and the lower-density portion is dissolved by etching to form a U-shaped groove along which the sheet is to be broken. Thus, the process of this invention ensures a by far more accurate finish than any known method relying upon mechanical scribing or thermal strain cutting. No shell-shaped chip, or horn-shaped projection is formed by the process of this invention, insofar as it employs etching instead of mechanical scoring. The dissolving action of an etching solution makes properly beveled edges on the broken surfaces and eliminates the necessity for any beveling job.
The temperature to which the sheet is heated is preferably lower than the dissolving temperature of glass, so that the sheet may not be affected adversely in flatness or shape. Two laser beams facing each other are preferably employed for heating both sides of the sheet simultaneously, so that two U-shaped grooves may be formed on the opposite side of the sheet from each other to make its cutting much easier. The cut edges are advantageously beveled on both sides of the sheet.
The cooling of the sheet is preferably carried out forcibly immediately after its heating, so that a high cooling rate may bring about an accelerated reduction in density which makes it possible to form a deeper U-shaped groove and thereby facilitate cutting. Any gas or liquid, or any gas-liquid mixture can be used as a coolant. Air is, however, preferred as a coolant for forced cooling, since it is inexpensive, does not have to be collected after use, is easy to handle and requires only a small apparatus. Any other gas, such as carbon dioxide, nitrogen or argon, requires a gas bottle, while a liquid coolant, such as water or oil, requires a pump and a collection tank, and in either event, the coolant requires a more expensive apparatus.
The groove preferably has a depth of at least 50 microns. It has been found experimentally that a groove having a smaller depth results in the failure of a glass sheet to be broken across its thickness or along the groove.
An aqueous solution of acid ammonium fluoride is preferably used for etching. Acid ammonium fluoride is suitable for use in forming a U-shaped groove because of its action of dissolving glass. An aqueous solution of acid ammonium fluoride obtained by mixing sulfuric acid with ammonium fluoride has a milder dissolving action and is, therefore, suitable for use in beveling the edges.
The sheet preferably has a linear expansion coefficient of 35xc3x9710xe2x88x927/deg. C or higher in a temperature range of 0 to 350 deg. C, since a higher coefficient brings about a greater specific volume.
The cutting line is preferably a closed curve having an ending point coinciding with its starting point. This invention is useful for cutting a glass sheet along either a straight line or a curved line, but is particularly useful for making a product by cutting a glass sheet along a closed curve, since a beautiful closed curve can be drawn by a single stroke of a laser beam if its position is numerically controlled. A true circle is, for example, a closed curve. Every glass disk cut out along a true circle from a large glass sheet is expected to have a very high accuracy of roundness. Thus, it is possible to produce disks of glass having a very high dimensional accuracy on a mass-production basis.
The laser beam is preferably inclined to a line normal to the surface of the sheet so that the groove may have a conical or likewise tapered surface. The conical or tapered surface facilitates the separation of any product from a large glass sheet and enables it to be separated without making any undesirable contact with the sheet, or base material and maintain its good quality without having any damaged or chipped edge portion.
According to a second aspect of this invention, there is provided a glass disk for a recording medium, the disk having an inner periphery and an outer periphery each in the form of a true circle, and being of glass having a linear expansion coefficient of 35xc3x9710xe2x88x927/deg. C or higher in a temperature range of 0 to 350 deg. C, each principal surface of the disk and each of its inner and outer peripheral surfaces having therebetween a corner formed by a part of a groove formed by applying a laser beam to the surface of a glass sheet before cutting out the disk therefrom, cooling the sheet and etching it with a solution containing fluorine.
Glass having a linear expansion coefficient as stated above permits the formation of satisfactorily deep U-shaped grooves along cutting lines by laser beam application, cooling and etching. The deep grooves facilitate the separation of the glass disk from the sheet. Each cut surface defining the inner or outer periphery of the disk has a pair of corners each formed by a beveled edge of a U-shaped groove. The positioning of the laser beam can be numerically controlled so that each of the inner and outer peripheries of the disk may have a high accuracy of roundness with a deviation in the order of only several microns. Thus, the disk of this invention is suitable for a recording medium owing to its high roundness and its beveled corners.